Hepatocytes were isolated from livers of male Sprague Dawley rats weighing 250 g and treated with various hypolipidemic drugs. Peroxisomal proliferation was assessed by increase in peroxisomal enzymes carnitine acetyl-transferase (CAT), carnitine palmitoyltransferase (CPT) and an 80K protein which was isolated by gel electrophoresis. Clofibric acid and ciprofibrate increased CAT activity about 2 fold in liver cells with smaller increase in 80K protein in 48-72 hr. Ciprofibrate was more potent than clofibric acid (EC50 for ciprofibrate = 50 MuM vs CC50 for clofibric acid = 300 MuM). Maximal increase in CAT activity induced by these two drugs individually were not additive. Ciprofibrate induction of CAT was blocked by cycloheximide indicating de novo synthesis of enzyme. Elevations in 80K protein by these drugs were similar to the increases in CAT. Both drugs induced [35S]methionine incorporation into protein as well as into various peroxisomal enzyme. RNA-synthesis as estimated by incorporation of 3H uridine was measured by these drugs. Drug induced proliferation of peroxisomes were also confirmed by electron microscopic examination. When hepatocytes were tested with clofibric acid, it increased mRNA synthesis coding for various peroxisomal marker enzymes such as catalase and CAT as demonstrated by translation of RNA in vitro using rabbit reticulocyte lysate and [35S]methionine containing amino acid mixture.